Cytotoxic T lymphocytes (CTLs) against virus-induced tumors are highly effective mediators of tumor-specific immunity in vivo. CTLs bearing the surface molecule CD8 recognize small (approximately 10-amino-acid) viral peptides that are presented in association with major histocompatibility (MHC) class I molecules at the surface of tumor cells. In the case of mouse tumors induced by the early region (E1) of human adeno-virus type 5 (Ad5), cloned CD8+ CTLs directed against a peptide derived from the viral nuclear oncogene product E1A can rapidly eradicate large established tumors, provided that interleukin-2 (IL-2) is given simultaneously. Similar findings have been reported for other virus-induced tumors. Conceivably, adoptive therapy with CD8+ CTLs could be extended to tumors that originate in other ways, including those arising through activation or mutation of cellular oncogenes.